1. Field of the Invention
The present invention relates to radio frequency (RF) amplifiers.
2. State of the Art
Presently, much attention and engineering effort is being focused in the area of wideband cellular telephony, i.e., xe2x80x9cthird generationxe2x80x9d (3G) cellular systems. Web-enabled telephones will increasing rely on such systems to deliver added value and make the mobile Web experience more compelling. An example of a wideband cellular technology is EDGE, a high-speed air interface that may be used within the existing GSM cellular infrastructure. Presently, the modulation used by EDGE is a variant of 8PSK, i.e., eight-level Phase Shift Keying. The data constellation in such a modulation scheme is substantially more complex than that used in current second-generation systems. The data constellation itself entails a dynamic range of 18 dB of transmit power. Power level adjustment requires an additional 27 dB of dynamic range, for a total of 45 dB dynamic range. For other 3G signals, such as UMTS, transmitter dynamic range exceeds 90 dB.
In the case of conventional linear power amplifiers, achieving such dynamic range is relatively straight-forward. Linear power amplifiers, however, exhibit very low efficiency, particularly at middle and low output power levels. For 3G cell phones, efficiency is a paramount consideration.
Furthermore, in EDGE and similar 3G technologies, the transition of the signal vector from one point in the data constellation to another point in the data constellation is required to pass near to the signal origin, defined by zero or near-zero output power. Precise control of amplitude and phase trajectories in this manner becomes problematic over the wide dynamic range described.
The desire for efficient power amplification has resulted in techniques such as the LINC (LInear amplification with Nonlinear Components) power amplifier technique. In the LINC power amplification technique (described, for example, in U.S. Pat. No. 5,990,738, incorporated herein by reference), a signal which has amplitude variations is generated by combining two signals which vary only in their relative phases. The vector sum of the two signals can represent any amplitude and phase.
Referring more particularly to FIG. 1, a LINC amplifier 10 amplifies two or more constant amplitude signals, which represent an input signal to be amplified. The LINC amplifier uses a signal separator 11 to split the input 12 into the two components 13, 14, which are constant amplitude, phase varying components. The LINC amplifier may be supplied a complex baseband digitally sampled signal 12. The baseband signals 12 can be a representation of multiple modulated carriers using any modulations. For simplicity, various details such as the need to convert from baseband to a higher frequency and the need to convert from digital into analog have been omitted.
Since the power stages 15, 16 do not have to deal with amplitude variations, it is possible to build an amplifier which will amplify signals linearly by using the two phase and frequency modulated components. The nonlinearity of the amplifiers is no longer a problem in the amplification of multiple signals or those containing amplitude variations, because the constant amplitude of the two components 13, 14 become constant amplified amplitudes as they are amplified by amplifiers 15, 16, while the phase of the components passes through the amplifiers with a constant shift. Although the nonlinear amplifiers produce distortion signals at multiples of the carrier frequency, these can be filtered off. The output signals from 15, 16 are vectorially combined in combiner 17 to produce the final output signal 18.
The vector diagram of FIG. 2, including FIGS. 2(a)-(e), illustrates the manner in which the output signals of FIG. 1 vary over time to achieve a particular signal transition. Here, for purposes of illustration, the LINC component signal vectors (solid lines) and the resultant signal vector (dotted line) are sampled along an example transition within a 16-QAM constellation. The resultant signal (central vector) is assumed to progressively decrease during a portion of the transition and then progressively increase to again reach the starting magnitude. As the resultant becomes small, since the signal components are of constant magnitude, the phase angle between the signal components approaches 180 degrees. In general, LINC requires phase shifters operable over a wide range.
In LINC, because the two component signals are constant amplitude, non-linear amplifiers may be used, Furthermore, because more efficient amplifier operation is achieved using non-linear (as opposed to linear) amplifiers, considerable efficiency gain may be achieved in the power stages. Nevertheless, in many instances, the two signals are in an antiphase or near antiphase relationship, with the result that most of the signal power is dissipated in the signal combiner. Also, in some instances, the signals may be required to undergo rapid phase rotation, substantially increasing component-signal bandwidth. For example, FIG. 3 is a graph illustrating the signal bandwidth effects of the signal transition of FIG. 2. An example signal transition may exhibit the frequency profile of the middle solid line. The advancing LINC component signal (triangles) begins with a higher frequency offset than the intended signal, and continues on to have a higher peak frequency than for the original signal. The following LINC component (squares) starts actually rotating in the opposite direction (negative frequency shift). Following the midpoint of the signal transition, the roles of the two LINC component signal reverses. Clearly, in an amplitude and angle modulated signal such as this, the block generating the LINC component signals must have greater bandwidth than the incoming signal alone. This increased bandwidth requirement can be a problem for wide bandwidth input signals.
Future 3G cell phones will require power amplifiers that exhibit high efficiency and a large dynamic range consistent with wideband operation, as well as precise control of amplitude and phase trajectories. In particular, amplifiers are needed that avoid the large power dissipation that occurs in the LINC combiner at lower output levels.
The present invention, generally speaking, provides a highly efficient RF power amplifier having a large output dynamic range. The amplifier avoid the large power dissipation that occurs in the LINC combiner at lower output levels. In general, this is achieved by using power-controlled switch-mode power amplifiers to vary output power at large power outputs, and reverting to LINC only at low output powers. Thus the present invention achieves the desirable combination of high efficiency at all output powers, while maintaining the fine output power control of the LINC method. More particularly, the power amplifier is based on a highly efficient structure in which amplitude modulation/power control of the output of an RF amplifier is achieved by operating the amplifier in switch mode and varying the power supply of the amplifier, as described more fully in U.S. patent application Ser. No. 09/637,269 entitled High-Efficiency Modulating RF Amplifier, filed Aug. 10, 2000 and incorporated herein by reference. Such an amplifier behaves quite linearly at high power but may exhibit non-linearity at lower output powers. Good linearity throughout a wide dynamic range, and particularly at low output power, may be achieved by phasing and combining the outputs of two (or more) such amplifiers. At high and medium power, the outputs are combined in-phase, allowing for low-loss, high-efficiency operation. At low power, the outputs are phased such that power subtraction occurs. In this manner, each amplifier may be operated at a power level that exhibits good linearity while producing an output signal of a lower power level that would normally fall within a region of substantial non-linearity.